This page is dedicated to the development of a converter of the data
port on a Cen-Tech caliper / scale to a more supported format.

In the process of adding a DRO to my 44142 combo lathe / drill machine
I found the YADRO project (http://www.yadro.de/).
I really liked the idea of this software DRO due to it's low cost and
flexibility. I proceeded to build up a set of hardware and ordered a
set
of Cen-Tech calipers to convert to scales from harbor freight (http://www.harborfreight.com/6-inch-digital-caliper-47257.html).
The nice thing about the calipers was they were inexpensive ($20 normal
$10 or less if you wait for a sale) and they had a few sizes in stock
that would cover my mill needs. BUT, when I finally hooked up the
scales
they did not work. Looking at the images at the YADRO site and my
scales they are identical on the outside. But, unfortunately the guts
must be different. The protocol the Cen-Tech uses is not the same as
the 2*24 (sylvac) or 7BCD supported by YADRO. My goal is to make a
adapter to convert from Cen-Tech (1*24) to 2*24 for use with YADRO and
many other devices out there.

News

4JUL10

!!!!!!!!!!!!! READ THIS FIRST !!!!!!!!!!!!!!!!!!!!!
Today I picked up a new 8" Cen-tech caliper for the Y axis of my mill
(finaly got around to taking the carage off to make the alters needed
to mount it). When I went to test it out it did not work with my
adapter. After a little investigation I found out it uses the true 2*24
protocall and works directly with the normal YADRO interface card!
Easy way to check which scale you have is to check the clock pin
frequency if it is around 65kHz it may be 2*24 if it is around 4kHz
more than likely it is the 1*24 format. The outward dimentions and pin
out are identical between the 2 scales. Inside there is a extra set of
pads for a watch crystal. (one of my new ones had it install the other
did not.)

27JUN10

I have finished the quadrature firmware. (click here for more info)
In the process of debuging this I found a error in the raw card
preventing it from being used for quadrature encoders. I have updated
the raw card and firmware for the cen-tech adapter so one raw card can
be used for both. Not I have not build any of the new raw cards so far.

21MAR10

Another update from John on Measumax scales.

It turns out that the datasheet which came with the
Measumax scale was wrong - they had the data and V+ lines swapped!
There
must be reverse polarity protection diode in series with the USB
connector to
the outside world. I only got the scale to work when using the internal
battery
and the "pull-up" resistor. It is a bit hard to describe in words (and
probably
not very interesting for you!) but the upshot is that when I swap the V+
and
data wires to the YADRO-Centech, it works perfectly - well, almost.
remember the
reverse polarity protection diode? It means that the 1.6V which the
YADRO-Centech card passes to the scale isn't enough to power the scale.
To fix
it, I put a 2.7V zener in series with the 5V line, and a pull-down to
Gnd (to
get the zener current up to around 5mA. I feed the resulting 2.3V to the
"Centech", which "sees" 1.65V after the reverse polarity protection
diode, and
everything is absolutely perfect! I made a couple of mods to the board,
using
the Q5 holes for the zener, and the R10 holes for the zener's resistor,
cut
a couple of tracks and used the component leads to form a couple of new
tracks.
It is quite neat - and no new holes needed to be drilled.

14MAR10

John sent this in:

I had two minor changes to make in order to
get it to work
with the intended scale (a "Measumax" brand):

It needed a pull-up on the data line. The
clock line was OK
- but the data line appears to be an open collector output, so a 10k
pullup
worked a treat. Actually, I'll modify the board slightly, to use R4 as
the
pullup - I'll cut the 5V feed which is supplied to the calliper, and
connect
the top of R4 to the 1.5V feed instead.

I had to omit Q5, and put a link between C
& E, since the
Vce drop of the transistor prevented the scale from working
properly.

So it sounds like with minor changes Measumax brand scale work under YADRO via this adapter as well. (Thanks John)

05DEC09

Adapter built up and verifed good!!
If anyone needs a raw card or programed micro just drop me a email!
Also I have 1 extra Yadro raw card available (minimum order was 2) asking $40 + shipping. Email me if interested.

21NOV09

Raw cards arrived! Will start assembling the first one when I get back after thanksgiving.

14NOV09

Scale to Yadro adapter finished and uploaded. First batch of raw cards on order now.
I have also made a achive copy of www.yadro.de in case it disapears
from the internet. (Last update was in 2006 and no responce fromt he
listed email address) If you need a copy just send me a email and I can
send it out.

16AUG09

Scale to RS232 adapter finished and uploaded

19SEP08

Created new page to cover the progress of the
converter.

1*24 protocol / Cen-Tech info

The protocol used by the Cen-Tech devices I have named
1*24 since it is one packet of 24 bits that repeats.

Every 4th bit the high is stretched so the cycle
takes 540us but the data IS valid over the entire high.

The data is sent from the caliper apx every 100ms.

Far
as I can tell there is NO remote control on this scale.

It
automatically falls asleep every so often but wakes up as soon as the
scale moves. When the scale wakes up it keeps the last unit setting and
0 location. (ie location is not lost). During sleep the LCD and data
port are shut down.

After a power cycle both the location and units are reset. 0 is placed
at the current location and units are set to mm.

Like many other scales the + side of the battery is connected to the
frame of the scale.

Cen-Tech to RS232 adapter

I have finished a hardware dongle based off a PIC16F628
that converts the Cen-Tech protocal into RS232. Also included in the
zip file is a windows perl script that will read the dongle connected
to a serial port and report back the readings.